At present, foamed-crosslinking type urethanes are widely used a cushioning material that is used for furniture, beddings such as beds, and seats for vehicles such as those for trains, automobiles and two-wheeled vehicles.
Although foamed-crosslinking type urethanes have excellent durability as a cushioning material, there is a problem that they easily get stuffy for its inferior moisture-water-permeability and air-permeability, and its thermal storage property. Since the foamed-crosslinking type urethanes do not have thermoplasticity, they have difficulty in recycling, and therefore they give significant damage to incinerators and need high costs in elimination of poisonous gas in case of incineration. For this reason, the foamed-crosslinking type urethanes are often disposed of by landfill, but it also causes problems that the landfill spots is limited because of the difficulty of stabilization of ground and the costs increases. Furthermore, although the foamed-crosslinking type urethanes have excellent workability, they may cause various problems such as pollution problems with chemicals used in the manufacturing process, residual chemicals after foaming and associated offensive odors.
Patent Documents 1 and 2 disclose network structures. They are capable of solving various problems associated with the foamed-crosslinking type urethanes and have excellent cushioning performance. As for the repeated compression durability properties, although those network structures have excellent performance with regard to the repeated compression residual strain with the 20000-time repeated compression residual strain being not more than 20%, they have a low hardness after repeated use with the 50%-compression hardness retention being only about 83% after repeated compression.
Network structures have been heretofore considered to have sufficient durability performance if the repeated compression residual strain is low. In recent years, however, it has been increasingly required to secure cushioning performance after repeated use with compression as requirements for repeated compression durability have become higher. However, in regard to the conventional network structures, it is difficult to obtain a network structure having durability performance which satisfies both the requirements of low repeated compression residual strain and high hardness retention after repeated compression.
Furthermore, as consumer needs in recent years, not only low-repulsion types but also high-repulsion types have been increasingly desired as characteristics of cushioning bodies. Patent Document 3 discloses a method for producing a low-repulsion cushion using a polyolefin-based thermoplastic elastomer. In this production method, a network structure having satisfactory low-repulsion characteristics can be obtained, but it is difficult to produce a network structure having excellent high-repulsion characteristics.